And so it's over. The last British troops are withdrawing from the city of Basra to regroup in the sprawling camp at Basra Air Station, the perfect launch pad for an exit strategy. Only last December, in one of his photo-op speeches to them, Tony Blair had declared: "Don't be in any doubt at all - the troops will stay until the job is done." But the job isn't done. It was never do-able. And there's even a certain symmetry about it. The mission will end, as it began, on the basis of a falsehood.

Two months before the New Labour landslide in 1997, the constitutional scholar Patrick Hennessy chaired a breakfast meeting of senior civil servants and businessmen, addressed by a senior figure in the Blair camp. This outrider of the project told them to expect a style of government that was less baronial and more Napoleonic. And so it proved. Our Bonaparte bestrode the world. And Iraq was his Waterloo.

Now he's gone. Not since disgraced members of the old Soviet politburo were airbrushed out of the group photograph on Lenin's tomb has a public figure vanished from public view quite so completely. But we are left to deal with the consequences. In a series of policy shifts and initiatives, Gordon Brown has staked out his own territory and made the past seem another country. The exception has been Iraq, where all he has done so far is appoint two ministers, John Denham and Mark Malloch Brown, who were with the rest of us in seeing it for the grievous mistake that it was.

In the four and a half years since the ill-fated invasion, I cannot remember a public argument being so decisively won on one side and lost on the other.

A new political season begins this week. The three main parties have elected new leaders since the war, and I know few MPs who voted for it who do not, in private at least, admit to having made a mistake. Now Brown must go further, make a clean break with the past and order the full and independent inquiry that has so far been refused.

The case for it is overwhelming. We need to understand why the warpath was chosen when diplomatic options were not exhausted; why there was a plan for war but not for peace; why the armed forces were sent to kick in the door of a sovereign state on the basis of a whim about regime change and a falsehood about weapons of mass destruction. And what lessons can be learned, so that never again do we park our foreign policy so unconditionally up the Potomac.

The case against an inquiry, which was always weak, had now all but vanished. The argument that it would endanger our troops, carried to its logical conclusion, would mean that the worse a mistake a government makes, the less it should be held to account.

And it is the soldiers' argument, so many dead and so many maimed to no purpose, that makes the case for an inquiry all the stronger. If the public had an idea of the scale of those injuries - the many hundreds disabled in body and mind - they would have risen up in anger long ago. The families know well enough. But the Ministry of Defence has successfully kept this knowledge from the wider public.

We also owe it to those whose country we invaded, and whose dead remain uncounted.

The inquiries we have had, led by Lords Hutton and Butler, were certainly no substitute for the inquiry that we haven't. Lord Hutton's investigation, whatever its quality, was limited to the circumstances surrounding the death of Dr David Kelly, and Lord Butler's to the accuracy and use of the intelligence on weapons of mass destruction.

Lord Butler's conclusions were actually damaging to the government: they would have been even more so if he could have been persuaded to write them in plain English rather than the silken understatements of his kind.

We must not be in denial any longer. If a people or a government is guilty of serious wrongdoing, on a scale from the discreditable to the shameful, then we know from experience that it becomes itself a victim of its actions. That was why the South Africans needed their Truth and Reconciliation Commission to lay the groundwork for the new South Africa. It was why the Bosnian Serbs held their own commission of inquiry into the Srebrenica massacre. It took them nine years, but they got there in the end. "We have reached historic perceptions," said the chairman of the commission, "and we will have to face ourselves."

The cases are different, but the principle is the same. We, too, will have to face ourselves. We need an inquiry to do for the war in Iraq what the Esher inquiry did for the Boer war - unflinching, far-reaching and unsparing of reputations.

· Martin Bell's book The Truth That Sticks is published this week. Listen to Martin Bell today on Newsdesk, our daily podcast, at theguardian.com/podcasts